The present invention relates generally to golf balls, and more particularly to golf balls having relatively thick multi-layer covers that are durable and resistant to cracks and fractures. In a further aspect, the present invention relates to golf balls having multi-layer covers in which each of the individual cover layers have the same, or similar, hardness.
In another aspect, the present invention relates to golf balls exhibiting particular PGA compression values. And, in yet another aspect, the present invention relates to golf balls having cores with certain coefficient of restitution values that are matched with such values of other components in the balls such that the difference between the overall coefficient of restitution of the ball and the coefficient of restitution of the core is relatively small.
Top grade golf balls sold in the United States generally comprise a central core with one or more cover layers formed thereover. A golf ball cover is particularly influential in effecting the compression (feel) and durability (i.e., impact resistance, etc.) of the resulting ball. Various cover compositions have been developed in order to optimize desired properties of the resulting golf balls.
FIGS. 1 and 2 illustrate a conventional golf ball 10 having a single cover layer 14 molded about a golf ball core 16. FIG. 2 illustrates (in an exaggerated view) stress lines 12 extending partially, or entirely across the thickness of the cover layer 14. Stress lines 12 typically result in a crack or fracture across the thickness of the golf ball cover. FIGS. 1 and 2 illustrate one problem that may occur when a very thick, single layer cover is formed about a golf ball core.
Although not wishing to be bound to any particular theory, it is believed that stress lines in a golf ball cover, such as stress lines 12 in cover 14, result from repeated strikes with a golf club, particularly drivers, and temperature effects. Stress lines often serve as initiation sites for crack or fracture propagation in a golf ball cover material. Such cracks or fractures, and their related stress lines, are undesirable in golf ball covers. Moreover, it is particularly undesirable for such stress lines and the resulting cracks or fractures to extend across the entire thickness of a golf ball cover since such damage significantly impairs golf ball performance. And, such cracks or fractures greatly reduce the durability of a golf ball cover.
When a multi-layer cover is employed, each cover layer traditionally has had a significantly different Shore D hardness than an adjacent cover layer in order to impart to the golf ball a particular desired combination of spin and distance characteristics. Although the use of a multi-layer cover configuration reduces the tendency of stress lines, and thus cracks and fractures, propagating across the entire thickness of the cover, such multi-layer arrangement of cover materials, each having its own particular set of properties and characteristics, has associated design and manufacturing problems.
For instance, in order to produce a multiple cover layer golf ball that exhibits a desired set of performance characteristics, it is necessary to design and anticipate an overall performance profile for the set of cover layers. This involves analyzing each of the individual cover layers and any and all effects between the individual cover layers. Even if such daunting design analysis is performed, the increased number of variables may lead to unanticipated difficulties in manufacturing or with the final product golf ball.
In addition, although, once again, not wishing to be bound to any particular theory, it is believed that although a multiple cover layer configuration may reduce the tendency for cracks or fractures that extend through the entire thickness of the cover, such configuration may lead to an increase in the number or frequency of fractures, particularly in applications in which the various cover layers constituting the multi-layer cover each have different physical properties such as hardness and flexural characteristics.
Accordingly, there is a need for an improved golf ball which is less susceptible to cracking or fracturing across the thickness of the cover than currently known single cover layer golf balls. And, there is a need for an improved multiple cover layer golf ball that is simpler to design and manufacture, and which is less susceptible to cracking or fracturing of one or more of the individual cover layers that constitute the multiple cover layer of the ball.
The present invention relates to new and improved golf balls which overcome the above-referenced problems and others. As a result, an object of the present invention is to provide a multi-layer golf ball with a durable, and preferably thick, cover. The multi-layers provide better durability by minimizing crack initiation or propagation through the formation of knit-lines. The cover layers may be of the same or different composition, but of similar hardness.
Another object of the invention is to provide a multi-layer golf ball having thick cover layers (i.e. overall cover thickness of at least 3.6 mm, 0.142 inches) exhibiting similar hardnesses. The multi-layer cover functions in a manner to produce superior durability. The cores of the golf ball can be solid, can be comprised of two or more layers, and can consist of thermoset or thermoplastic materials. Alternatively, cores can be wound, consisting of solid or liquid centers. One or more of the core and/or cover layers can contain a density reducing or increasing filler material.
A further object of the invention is to provide a golf ball having improved distance than conventional golf balls with a comparable coefficient of restitution (i.e., C.O.R.).
Yet another object of the present invention is to provide a golf ball having a low spin rate and excellent distance on full shots, combined with a soft feel when the ball is used for chipping and putting.
Yet a further object of the invention is to provide methods of making golf balls of the type described above.
In one aspect, the present invention provides a golf ball including a solid core having a coefficient of restitution of at least about 0.650, and a multi-layer cover having a thickness of at least about 3.6 mm (0.142 inches) and a Shore D hardness of at least about 60.
In another aspect, the present invention golf ball includes a core comprising one or both polybutadiene and natural rubber, such that the core has a coefficient of restitution of at least about 0.650. The golf ball also comprises a cover including an ionomer and having a thickness of at least 3.8 mm (0.150 inches) and a Shore D hardness of at least 60, such that the ball exhibits a coefficient of restitution of at least 0.770.
The present invention in a preferred form provides a golf ball comprising a core and a first cover layer comprising a resin composition and at least one part by weight of a filler material. The first cover layer surrounds the core. The golf ball further comprises a second cover layer comprising a resin composition surrounding the first cover layer, the second cover layer having a different overall composition than the first cover layer. The difference between the Shore D hardness of the first cover layer and the Shore D hardness of the second cover layer is 2 or less.
Another preferred form of the invention is a method of making a golf ball. The method comprises the steps of obtaining a golf ball core, forming a first cover layer over the core, and forming a second cover layer around and in contact with the first cover layer. The cover materials are selected and layers formed such that the difference between the Shore D hardness of the first cover layer and the Shore D hardness of the second cover layer is 2 or less.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.